Calcium dicyanamide and process for the preparation thereof



D. E. NAGY CALCIUM DICYANAMIDE AND PROCESS FOR July 31, 1951 THE PREPARATION THEREOF Filed May 13, 1950 ow 0.? Ow. om oO.

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OQ mb OF mw O@ mm. Om m* 0.7 mm. Om. mN ON m; O` m O \|.W-Q\% \.\U \QDM4 &\ .w 2L. IUQSTQX .QQ Q10 .w-%\\b\ vb\ S u MVG l l' r/ .vQ\n,\Q Nb bu VG INVENTOR ZM ATTORN EY Patented July 31, 1 951 CALCIUM DICYANME AND PROCESS FORv THE' PREPARATION THEREF y l i' Daniel E. Nagy; Stamford, bonn.,uassignouto" American Cyananiid Company, N. Y., a corporation ofzlyiaine HVNew' York;

This invention'. relates to calicumv dicyanamide and its dihydrate, and to-amethoid .for the preparation; of these materials.

In the past ithas been'. proposed to prepare sodi.-y um. dicyanamide'. by reaction. of disodiumvv cyan.- a-mide withl cyanoge'n bromide.. Thisreactio'n was disadvantageous. for anumbe'ro'f reasons.

In'` the Vi'lrst place', disodium cyanamidez is not an article of commerce and ispreparedfrom call- Icium` cyanamide by a series of delica-te and. in,- efficient reactions.` Secondly, yields of sodiurn. dicyanamide werelfound to `bepoor. Thirdly, it was found thatv during the.y main reaction leading/.to the formation of sodiuml dicyanamidea large proportion of 'sodium' hydroxide is `formed by the sidev reaction` NaNCN rcoL \NCN Neonv and that' this sodiumL hydroxide"A reacts readily with the cyanogenbromide to form a large proportion of sodium cyanate. Aside from the quesb tion of waste; of materials, the sodiuml cyanate thus formed; like sodium dicyanamide, is quite soluble, and therefore the presence of the large amount ofy soluble sodium' cyana'te Vin the final crude' solution ofsod-ium dicya-namide presented a serious purioationproblem. Finally, sodium dicyanamide, as such, was found to be insulcientlyuseful, inthat' it wasJ nota suitablearaw'material for the direct preparation of the numerous otherv soluble salts of' dicya-namidea The direct preparation of these soluble-salts' involves-a double decomposition reaction, :andi nov industrially practical wayA of precipitating `sodium is known.

The surprising 'discoveries' havenow 'been made that calcium dicyanamide may be prepared di'- rectl'yy from calcium cyanamide and cyanogen chloride or bromidein one'simpl'e step, and that even the crude fertilizer grade of calcium 4cyanamide gives yields' of the desired product in this reaction which are substantially the same as when the pure 'equivalent 'of crude calcium cyanamide employed. Ithas further 'been found that calcium dicyanamide is a compound of amazingly high solubility, Candthatit is vabout-three times as soluble as sodium dic'yanami'de'. Asis' shown iin the drawing, this solubility isaboutf88% at room temperature, and `surprisingly does 'not' change significantly over the range 'C.75 C'. This fla-t solubility curve lpermits the eici'ent separation'of calcium difcyanamide-from a ywidejfvariety of soluble salts: having sharply inclined H'solubility curves by selecting temperature ranges'and cone centrationsf appropriate for this purpose;l

'f cyanamide is. slurriedin l water.Y

thet calcium hydroxide.;thus,formed in the presence of calcium acid cyanamide, unlikethe sodil umihydroxide iormedinEAquation 1, is virtuallyl inert to cyanogen. chloride under'the conditions. which are optimum for the formation of calciuxri dicyanamide, that only a negligible proportion o`fcalcium cyanateforlns; and that therefore the purification problem; noted'above in the case oi the sodium reaction is virtually non-existentlwhen calcium` cyanamide isemployed as. the raw material.r Asa result., nearly theoretically high yields of. the desired productV a-re achieved. Finally, it has been found. thatcalcium dicyanamide is an ideal materialhfor the direct preparationof the sodium and other solublesaltsot dicyanarnide, in thataddition of. the sulfate lor carbonate .of the metal which it desiredto introduce causes. immediate and complete precipitation of all, the calcium which was present. as calcium dicyanamide. For example,y additionof ammonium sulfate,l toY calcium dicyanamide solution` causes pre.- cipitation of the calcium as calcium sulfate, and alsolutionpff pure ammonium dicyanamid'e. re,- sults. Thisresultis vimpossible when sodium, di.- cyanamideisemployed as. the starting material.

g Calcium dicyanamide. has distinctive properties as-compared with those .possessed by the salts of dicyanamidewith monovalent metals such as sodium.. Forexarnple, the monovalent salts polyl merize readily V*when strongly heated and forni l thev soluble tri-metallic:derivatives of tricyan 5: l v .y L..;Cyanogen chloride: ia-.then added to this slurry,

omel-ami-ne..- Calcium .dicyanamide, however, whenheated-.underfthe same conditions also polymerizes,1 but the materialV thus obtained is amore phous and insoluble.

A further difference between the two is that calcium .dicy-anamide forms a stable hydrate, whereas the sodium.` and potassium dicyanamides formnohydrates whatever;

r According to the. present `invention, calcium cy anami'de is rstslurried ins-water.4 This leads .to the formation of calcium acid cyanamide:

whereupon calcium dicyanamide and hydrochloric acid are produced:

(3) H /CN il: oN N\ Y oN ca zoNoi on 2Ho1 CN N-CN N/ During this main reaction the side reaction noted above takes place leading to the formation of a small proportion of calcium cyanate:

(4) Ca(OH) 2-l-2CNCl-)Ca (CNO) 2+2HC1 The hydrochloric acid formed during reactions (3) and (4) reacts with the calcium hydroxide formed in the course of reaction (2).

When cyanogen chloride is employed as shown above, reaction (4) proceeds only to a negligible extent (3 %5%) and therefore upon completion of reaction (3) a crude solution is obtained containing principally calcium dicyanamide and calcium chloride in approximately equimolecular proportions. f

` In performing reaction (2) it is not necessary to use pure calcium cyanamide or pure calcium acid cyanamide. Instead, ordinary crude fertilizer grade calcium cyanamide may be employed and with this material without any previous purification, yields of more than 90% have been obtained.

It is surprising that crude calcium cyanamide may be employed so successfully in the reaction. Crude calcium cyanamide contains only about 60% calcium cyanamide. The balance is a complex mixture of organic and inorganic compounds and carbon. It could not be predicted that merely contacting a slurry of such a grossly impure composition with cyanogen chloride, acidifying, and concentrating the resulting solution would give a precipitate of calcium dicyanamide of over 90% purity in yields of about 90%. It could not be predicted that the calcium dicyanamide and the calcium chloride present in the crude solution would almost completely salt out or suppress the solubilization of the large number of impurities present in a crude calcium cyan- .amide slurry.

It is a surprising and advantageous feature of .the instant invention that the impurities present in crude calcium cyanamide do not appear to react during the instant process and that'they are substantially insoluble inthe final reaction solution. As a result, they may be readily separated by iiltration or decantation, leaving a solution containing only calcium dicyanamide, calcium chloride, and calcium cyanate as significant constituents. I

It is not necessary to use a preformed calcium cyanamide, although this is the preferred material from the point of view of cheapness. It is possible to produce the equivalent of this material by reacting an aqueous solution of free cyanamide with aqueous calcium hydroxide according to the equation N-CN The solution thus obtained contains calcium acid cyanamide and corresponds to the solution formed in reaction (2), and may be employed in the reactions outlined above as an equivalent thereof. Use of free cyanamide is ordinarily not advantageous, however, one reason being that in its anhydrous form it must be kept under refrigeration, and in aqueousl solution this material is unstable and tends to polymerize when present in a concentration greater than about 20%. As a result, with this material it is necessary to handle and process an undesirably large proportion of water.

In performing reaction (2), the time of slurrying may be very brief or even may be omitted entirely. Yields of well over are obtained after onlyfa few minutes of stirring, and the improvement continues gradually as stirring is continued over a period of about an hour. This high initial reactivity towards cyanogen chloride is very surprising, as crude calcium cyanamide is an insoluble material which is ordinarily coated with a water-repellant material.

The reaction may be run at any moderate temperature but at temperatures above 5()o C. disadvantageous yields are obtained due, in part, to undesirable side reactions involving the cyanogen chloride. The reaction maybe conducted below 0 C., but in that range the speed of the reaction becomes undesirably slow. I prefer to operate in the range oi 20-30 C. which makes it possible to conduct this reaction with high yields in ordinary equipment without any need for heating or' for appreciable cooling of the reagents. The cyanogen chloride is passed into the aqueous medium at a rate suciently slow that escape of cyanogen chloride is reduced to a minimum, in accordance with best engineering practice. When a closed reaction vessel is used, cyanogen chloride vapor may be added above the surface and is readily absorbed thereby. In adding the cyanogen chloride in this way, the surface should be stirred. The speed of addition is not critical. The cyanogen chloride is usually added over the period 1.5-3 hours, the slower rate of addition apparently slightly increasing the yield of product.

It is a particular advantage of this invention that ordinary steel equipment may be used, and that no particular design of apparatus is necessary.

When less than one mol of cyanogen chloride is allowed to react with the reaction mixture per mol of calcium cyanamide and/or calcium acid cyanamide present, an incomplete reaction will result from which, however, a yield of calcium dicyanamide can be obtained. It is not advantageous to react more than about one mol of cyanogen chloride per mol of calcium cyanamide and calcium acid cyanamide present because a greater excess does not further the desired reaction.

When crude calcium cyanamide is the raw material, a. considerable proportion of insoluble material is present after completion of the reaction. It should be removed by filtration or by decantation.

It is a further particular advantage of the present invention that the above-described crude solution of calcium dicyanamide, after removal of anyinsolubles, as stated above, and neutralized or made slightly acid7 is. very stable and can be stored or used without further processing as a valuable industrial rawmaterial. For example, it may be reacted at about atmospheric pressure with hydrogen sulfide forming iirst cyanothiourea and then dthiobiuret. Calcium dicyanamide di- The almacen hydrate itself has proveduniquaas a most eilective iluidizing agent for starch. adhesives, in some; instances being more than twice as elective as. sodium dicyanamide for this purpose. A much. broader utility. is indicated by the fact that theV solution readily reacts with. amines to form a wide variety of cyanoguanidines and disubstituted biguanides.

Upon completion of'reactions of the latter type, frequently compounds of low or moderate solubility are` obtained which precipitate or whichv can be recovered in substantially pure form by concentrating the solution. The thus-prepared compounds of low solubility readily precipitate, leavingv the very soluble calcium chloride in the solution.

Calcium dicyanamide dihydrate may be Vrecovered from the above-described crude solutions of calcium dicyanamidein severall Ways.

One simple meanslis to concentrate the solution. whereupon calcium dicyanamide dihydrate precipitates in substantially pure form. This precipitation is greatly assisted. by the presence of the calcium chloride, which exerts a pronounced salting out effect. By this means, recoveries of about 90% of calcium dicyanamide in solution is effected in a form about 90% pure. The product obtained can be further puriiied.

The above-noted salting out effect was quite unsuspected, because when a solution containing equimolecular proportions of potassium dicyanamide and potassiumv chloride, and potassium cyanate was concentrated, the potassium chloride was salted out leaving the cyanate and dicyanamide in solution.

A more costly method Vis based on the precipitation of a metallic dicyanamide by a double decomposition reaction. According to this procedure, with the crude ltered solution neutral or slightly on the acid side to prevent precipitation of zinc hydroxide, one equivalent of a strong solution of a soluble zinc or copper salt is added such as a 30% ZnClz solution. Zinc dicyanamide forms and precipitates at once with only slight coprecipitation of impurities; its solubility is less than 4 g. per liter. A solution of calcium dicyanamide more than 90% pure is obtained by reacting an aqueous slurry of the thus-precipitated zinc dicyanamide salt with calcium hydroxide. This results in the formation of insoluble zinc hydroxide which may be` removed from the aqueous calcium dicyanamide solution. Pure calcium dicya-namide dihydrate may then be recovered by evaporation of the resulting solution.

The above-described pure solution of calcium dicyanamide is a valuable means for the rapidi and simple preparation of solutions` ofV a wide, yariety ofk other salts of dicyanamide: insubstantially pureform. For example, addition of one mol of ammonium carbonate or sulfate immediately yields a solution of the otherwise difficult to prepare ammonium dicyanamide. Further, while calcium dicyanamide cannot be prepared from sodium dicyanamide, the reverse is easily achieved by adding: sodiumA carbonate or sulfate as described aforesaid to a solution of calcium dicyanamide.

Calcium dicyanamide dihydrate crystallizes in a columnar habit with the development of domes. and pinacoids, has a cleavage parallel to the b axis, and basa positive optic sign. The refractive indijcersfare as follows: e=1.405l '0.005'; .480":20003`g .and y is' approximatelyl 1.82. ap,parentopticaxialv angle asobserved in air is 86, and the true optic axial angle-*computed therefrom-$552 Theoptic axialplane or prin-f water to the surprising extent of about 87 parts;

per 100 parts of water.,i at 20 C., 89 parts at 25 C., 100 parts at 50 Ci, and'V 89 parts at 75 C.

The following examples illustrate the practiceV ofthe invention and are not to be. deemed in limitation thereof. The parts are by Weight unless otherwise noted.

Example 1 Reactants Parts I' Cyanogenchloride 6114 l.0 Gyanamide (18% aqueous)` 239 1.0 Calciumhydrox' e; 90 1.2. Water 20D IL'I The calcium hydroxide is carefully added to` the agitated cyanamide solution,A and any exothermic tendencies are overcome with cooling. The cyanogen chloride is slowly added' to this vigorously agitated reaction mixture over two hours, and it is readily absorbed. The temperatureof this reaction is not allowed to rise above substantially 30 C. Upon completion` of the. addition of cyanogen chloride the reaction mixture is no longer exothermic. The reaction mixture is neutralized with a small amount of hydrochloric acid and filtered to remove any insoluble materials. The clear ltrate is concentrated and calcium dicyanamide dihydrate precipitates and is recovered.

The water ofY hydration is removed either byheating the calciumY dicyanamide dihydrate toV substantially 100 C., or by drying it at a lower temperature under vacuum.

Eample 2 Molar Reactants Parts Ratio Gyanogenchloride 6l. 4 1. 0 Crudecalciumcyanamide (58% CaNCN) 137. 5 1.0 Water 500 27. 8

thoroughly, and' this moist cake is slurried with aseaeec calcium hydroxide. in a minimum amount of water. The resulting zinc hydroxide is removed from .the aqueous solution containing substantially calcium dicyanamide. This solution is concentrated to produce crystalline calcium dcyanamide dihydrate which after recovery has physical properties identical with those described above.

A' repetition of this experiment is run in which the aqueous slurry of crude calcium cyanamide is stirred for about one hour prior to the addition of the cyanogen chloride. The latter is added at substantially 20 C. at the rate of approximately one mol per 1.5 hours. The calcium dicyanamide obtained is identical with that obtained above.

This reaction is run as described in Example 2. The reaction temperature is maintained at substantially 20 C., and the cyanogen chloride is added over a period of 1.6 hours. The calcium dicyanamide is isolated by precipitating the dicyanamide as the copper salt by the addition of substantially stoichiometric amounts of copper nitrate solution. The insoluble copper dicyanamide is iiltered, washed thoroughly with Water, andA reslurried With a stoichiometric amount of calcium hydroxide in a minimum of Water. The copper hydroxide is removed and the clear aqueous filtrate containing substantially calcium dicyanamide is concentrated to yield crystals of this material having physical properties identical with those reported in Example 1.

The thick calcium cyanamide slurry is stirred about ten minutes prior to the addition of cyanogen chloride. This treatment serves to wet thoroughly the calcium cyanamide particles so that a satisfactory reaction with cyanogen chloride may be had. The cyanogen chloride is added at a temperature range of 24-26 C. over a period of three hours. When the reaction is completed, as indicated by the spontaneous drop in temperature of a couple of degrees, the mixture is filtered to remove insoluble materials. The clear iiltrate is adjusted to pH With dilute hydrochloric acid and concentrated until substantially all the calcium dicyanamide therein has precipitated. lIts properties are those recorded in Example 1.

Ercample 5 The water is mixed with the calcium cyanae'. mide and the resulting thick slurry stirred slowly for ten minutes at 25 C. The cyanogen chloride is added as a gas over the surface of the slurry in three hours at 24-26o C. The mixture was made slightly acid and ltered. A strong solution, of Zinc chloride is added to the filtrate until precipitation of Zinc dicyanamide ceases. The yield of zinc dicyanamide based on the weighty of the cyanogen chloride is When dry, the zinc dicyanamide has a purity of about 96%.

Example 6 The above experiment was repeated using equimolecular proportions of reagents, a reaction temperature of 20 C., a stirring time of one hour, and three hours for the addition of the cyanogen chloride. A 91% yield based on the recovery of the zinc salt was obtained.

Example 7 The following procedure is particularly adapted to the large scale preparation of calcium dicyanamide dihydrate.

In a closed steel kettle equipped with a turboagitator and cooling coil 178 parts of crude calcium cyanamide containing about 60% of crude commercial calcium cyanamide are slurried with 474 parts of Water, a temperature of 22S-25 C. being maintained by means of passage of tap water through the cooling coils. With the slurry at this temperature-86 parts of cyanogen chloride are admitted into the kettle above the surface of the slurry at the rate of 17-20 parts per hour. The pressure does not exceed about 5 lb./in.2. Uponv completion of the addition of cyanogen chloride, 18 parts of carbon dioxide vapor are slowly admitted which react with the excess Ca OID2 present; The pressure rises to about `20 pounds per square inch with the addition of the carbon dioxide. The contents of the kettle are passed through a lter press. The yield of calcium dicyanamide in the filtrate is over 90% of a calcium dicyanamide contained thereon, based on the cyanogen chloride added. The pH of the nitrate is adjusted to 5. The solution is agitated for one hour, and then neutralized with sodium hydroxide. The resulting solution is concentrated at reduced pressure at a temperature below 50 C. until precipitation of calcium dicyanamide is substantially complete, the pH of the solution being maintained at '7. The precipitate is dried at '70 C. and is calcium dicyanafmide dihydrate of 91% purity with an over-all yield of 89%.

This application is a continuation-in-part of ACa -N CNa which was disclosed and claimed therein, was described therein as calciuml dicyanimide. In the present specification and claims the spelling of the Word dicyanimide has been changed to dicyanamide to make the spelling of this radical conform to the preferred usage of the chemical art.

9 I claim: 1. A member of the group consisting of calcium dicyanamide NC\ /CN N-Ca-N NC CN and its dihydrate.

2. Calcium dicyanamide having the formula NG CN NO CN 3. Calcium dicyanamide dihydrate having the formula No CN NC ON 4. A method of preparing calcium dicyanamide from calcium cyanamide which comprises reacting calcium cyanamide and cyanogen chloride in an aqueous medium at a temperature between and 50 C'. until an aqueous solution of calcium dicyanamide is obtained, and recovering the calcium dicyanamide from said aqueous solution.

5. A method of preparing calcium dicyanamide from crude calcium cyanamide which comprises mixing crude calcium cyanamide with sufficient water to form an aqueous slurry, contacting said' aqueous slurry with cyanogen chloride and maintaining the reaction mixture at a temperature between 0 and 50 C. until an aqueous solution of calcium dicyanamide is obtained. and

10 filtering the said aqueous solution to remove insoluble impurities.

6. In the manufacture of calcium dicyanamide from calcium cyanamide the step which comprises reacting cyanogen chloride and calcium cyanamide in an aqueous medium at a temperature between 0 C. and 50u C. until an aqueous solution of calcium dicyanamide is obtained.

7. As a new and useful composition of matter, an aqueous solution containing calcium dicyanamide NO ON dissolved therein.

DANIEL E. NAGY.

REFERENCES CITED The following references are of record in the le of this patent: 

2. CALCIUM DICYANAMIDE HAVING THE FORMULA
 4. A METHOD OF PREPARING CALCIUM DICYANAMIDE FROM CALCIUM CYANAMIDE WHICH COMPRISED REACTING CALCIUM CYANAMIDE AND CYANOGEN CHLORIDE IN AN AQUEOUS MEDIUM AT A TEMPERATURE BETWEEN 0* AND 50* C. UNTIL AN AQUEOUS SOLUTION OF CALCIUM DICYANAMIDE IS OBTAINED, AND RECOVERING THE CALCIUM DICYANAMIDE FROM SAID AQUEOUS SOLUTION. 